There are several processes known for the preparation of manganese dioxide. It is known that manganese (III) oxide will disproportionate to manganese dioxide and manganese (II) ion in the presence of acid. However, there are many competing side reactions involved which substantially interfere with the product yield. Also, any change in reaction conditions from prior art procedures has often drastically reduced the product yield. Therefore the disproportionation of Mn (III) has not often been relied upon in the synthesis of MnO.sub.2. The process also has the disadvantage of high cost.
A second process involves the reaction of a soluble manganese (II) salt with base to precipitate manganese (II) hydroxide. The manganese (II) hydroxide is then oxidized in the art to form MnOOH where of course the manganese now has a valence of three. The MnOOH is then air-oxidized to manganese dioxide with water as a byproduct. This process has the disadvantages of being very time-consuming and being more costly than the first.
A third method of preparing manganese dioxide is by the thermal destruction of manganese (II) nitrate salt at a temperature of 350.degree. C. However, this process has the disadvantage of producing beta-manganese dioxide which does not have sufficient depolarization strength for use in modern batteries.
In one prior art process described, for example, in French patents 1,306,706 and 1,525,333, the disproportionation of the trivalent manganese is carried out with sulfuric acid or the like. The trivalent manganese disproportionates, in the presence of the acid, to tetravalent manganese (i.e. manganese dioxide) and soluble divalent manganese. In this process it is possible to obtain a theoretic conversion of only 50% of the trivalent manganese (e.g. Mn.sub.2 O.sub.3) to synthetic manganese dioxide.
The formula for the disproportionation reaction is represented as: ##STR1##
The yield is generally considered to be somewhat higher because of the inclusion of water molecules in the gamma structure. In this process a precise control of concentration and quantity of acid, of reaction temperature and treatment time is extremely important since otherwise reductive side reactions interfere with the disproportionation and with the value of the product. The processing when carried out properly, yields a manganese dioxide of a gamma structure as is particularly desirable for use in primary batteries.
A conventional process of another type (see LEHRBUCH FUR ANORGANISCHE CHEMIE, 80, Aufl., S. 817 ff.) precipitates the manganese-II-hydroxide from its manganese salt solution with alkali-metal hydroxide with oxidation by atmospheric oxygen first to the manganese oxide hydrate. The manganese oxide hydrate undergoes further oxidation and splitting of water at elevated temperature to the manganese dioxide. Oxidation with atmospheric oxygen occurs slowly. The reaction can be represented as: ##STR2##
The last-mentioned process is more expensive than the first.
The third process for the recovery of synthetic manganese dioxide relies upon the thermal decomposition of Mn(NO.sub.3).sub.2 salts which, when carried out at 350.degree. C, yields manganese dioxide with a beta structure having little utility in batteries because it has pure depolarization capability.